1. Field of the Invention
This invention relates to thread construction used in a length measuring instrument such as a micrometer and a method of manufacturing the thread construction.
2. Description of the Prior Art
In the length measuring instruments and the like, there are adopted a number of highly accurate thread constructions. The reason why so many highly accurate thread constructions are used is that, when used in a driving portion of a micrometer for example, the highly accurate thread constructions are not only effective in securing the measuring accuracy but also convenient in obtaining a given measuring force. In short, the positional relationship between two articles operationally associated with each other can be fixed at a desirable position. On the other hand, however, the thread constructions, being subjected to the measuring force, are, so to say, the most easily worn portions. In consequence, necessity is voiced for the increased wear resistance with the initial accuracy being secured.
Recently, there has been proposed the utilization of ceramics material in the measuring instruments as viewed from the advantages thereof in wear resistance, light weight, low thermal expansion, electrical insulation and the like. However, no ceramic material has been used in the length measuring instruments such as a micrometer. This is because it is very difficult to obtain from the ceramic material the highly accurate thread construction which satisfies requirements of workability and economics. The reason is that, in the case of ceramic material, the conventional processes of working upon metals are not good on the ceramic materials as viewed from the high mechanical strength thereof. In consequence, in the case of ceramic material, the working should be reliably done by grinding as viewed from the question of accuracy. However, in that case, at present, there does not exist a grinding machine capable of grinding internal threads having a small diameter, e.g., a diameter of 12 mm or less, so that the thread construction, i.e., the thread construction of external threads and internal threads mating with each other cannot be obtained from the same material. On the other hand, if the thread construction has a large diameter, then the measuring instrument will have decreased practical values from the viewpoint of accuracy, economics and operation because of the characteristics of the measuring instrument.
In consequence, as an example of the adoption of the ceramic material in the measuring instrument, it has heretofore been limited to the construction of the main and auxiliary scales which are slidable on each other as seen in the measuring instrument disclosed in Japanese Patent Kokai (Laid-Open) No. 19604/82 for example, and moreover, such a problem is developed that a separate means should be added thereto to secure a predetermined measuring force, thus resulting in lowered practical value.